The prevalence of asthma and allergic diseases has been dramatically increasing for the last few decades and has reached epidemic proportions in industrialized countries. Current therapy is not preventive or curative. Thus, novel therapeutic strategies are urgently needed. Histamine-releasing factor (HRF) can activate mast cells and basophils in an IgE-dependent manner. As its secretion was found in bodily fluids during late-phase allergic reactions, HRF has been implied in allergic diseases including asthma. How HRF is involved in allergic reactions had remained enigmatic for two decades. However, our 2012 study changed this situation by identifying a subset of IgE and IgG molecules as HRF receptors; mapping of the immunoglobulin (Ig) Fab-binding sites within the HRF molecule led to the discovery of HRF sequence-based inhibitors, N19 and H3 peptides, which competitively blocked HRF-Ig interactions; administration of these inhibitors drastically reduced airway inflammation in mast cell- and IgE-dependent models of asthma; intranasal administration of nave mice with HRF caused airway inflammation in an FcRI (= high-affinity IgE receptor) and mast cell-dependent manner. In this project, to gain more structural insights into HRF-Ig interactions, (i) we will identify the molecular signature of HRF-reactive IgE and IgG molecules, and (ii) analyze structural and dynamic features for HRF-Ig interactions at atomic and submolecular levels by X-ray crystallography and NMR spectroscopy. If HRF inhibitors should ever be used in a clinical setting, they would initially be tested on patients with abundant HRF-Ig interactions. To mimic such conditions, (iii) we will test whether and what HRF inhibitors are effective to treat airway inflammation in transgenic mice overexpressing HRF in the lung or HRF-reactive IgE. Several HRF-related pseudogenes are present in human and other mammalian genomes. Therefore, (iv) we will test whether their gene products affect HRF functions. Knowledge to be gained through this project will enrich our understanding of HRF-Ig interactions and the pathogenic roles of HRF-Ig interactions in asthma models, which will be crucial for our future clinical study.